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Javiera Morales, Ximena Besoain, Italo F. Cuneo, Alejandra Larach, Laureano Alvarado, Alejandro Cáceres-Mella, and Sebastian Saa

Excessive nitrogen (N) use in agriculture has been associated with increased severity of the damage caused by Phytophthora species. In this study, we investigated the impact in vitro and in vivo of N about Phytophthora cinnamomi. The preliminary in vitro assay showed the effect of different N sources on the mycelial growth of P. cinnamomi. This assay indicated that ammonium nitrate (NH4NO3) and ammonium sulfate [(NH4)2SO4] allowed for greater control of P. cinnamomi mycelia in comparison with calcium nitrate [Ca(NO3)2] and potassium nitrate (KNO3) when used with 1000 ppm N. The in vivo assay showed the severity of P. cinnamomi in 5-month-old Juglans regia saplings grown under greenhouse conditions. We selected NH4NH3 as the source for N for the greenhouse assay, considering the inhibitory effect on the ingrowth of P. cinnamomi and the intensive use of this fertilizer in agriculture. Walnut saplings were fertilized with 0, 35, 70, 140, 210, and 1050 ppm N and were inoculated with zoospores of P. cinnamomi 45 d after the application of nitrogen treatment (DAA). They were harvested at 90 DAA. We found that a 70-ppm N fertilization reduced the development of P. cinnamomi, resulting in lower root and canopy damage indices (DIs) than the unfertilized inoculated treatments and fertilized treatments greater than 140 ppm. The results of the in vitro and in vivo assay agree that increased N concentrations were associated with reduced mycelium growth of P. cinnamomi, providing further evidence that N fertilization can mitigate this disease. Greater root and canopy damage was observed in saplings fertilized with 1050 ppm N, regardless of whether they were inoculated with P. cinnamomi, as a result of N phytotoxicity (verified through foliar analysis). In contrast, inoculated and unfertilized saplings (N0) also showed high root and canopy DIs associated either with the inoculation with P. cinnamomi or the no fertilization treatment. We postulate that 70 ppm N is the best fertilization rate for J. regia saplings because the positive effects of N on growth are maximized and the damage caused by P. cinnamomi is mitigated.

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Aude Tixier, Adele Amico Roxas, Jessie Godfrey, Sebastian Saa, Dani Lightle, Pauline Maillard, Bruce Lampinen, and Maciej A. Zwieniecki

Temperature is assumed to be the principal regulatory signal that determines the end of dormancy and resumption of growth. Indirect evidence that stem temperature interferes with phenology comes from the common orchard practice of painting stems to protect them from disease. This work studies the effects of application of white paint to the stems of persian walnut (Juglans regia) trees on winter stem temperature, carbohydrate content, and spring phenology. Painting bark resulted in the delay of budbreak by several days, higher nonstructural carbohydrate (NSC) concentrations in the bark and wood of painted extension shoots and changes in the spatial gradients of NSC during budbreak. The demands of maintenance respiration exceeded mobilization from local carbon pools during bud development suggesting a potential role of carbohydrate transport during spring budbreak in persian walnut. Painting provides an exciting perspective for mitigating effects of milder winter in orchards. The effect of reducing diurnal and spatial temperature variability limits early budbreak, NSC depletion associated with intense maintenance respiration, freeze–thaw cycles and frost dehardening.